This invention relates to semiconductor diode lasers and diode array lasers, e.g. phase locked array lasers, and more particularly to a method and arrangement for ease of alignment in providing efficient optical feedback to the emitting aperture of the laser while insensitive to limited misalignment between the laser and its external optical cavity feedback resonator. The alignment method of this invention also has application with a phased laser array to bring about and maintain fundamental supermode operation of phase locked array lasers.
It is known in the art to provide an external optical cavity with feedback means in the form of an external mirror wherein the output beam from a light source, such as a laser, is reflected back into the emitting region of the laser to provide stimulation emission of the laser. Such feedback means may be more sophisticated in the case of stabilizing the far field dominate lobe so that no beam shift occurs during continuous laser operation. See, for example, U.S. Pat. No. 4,656,641.
An article of E. M. Belenov et al entitled, "Methods for Narrowing the Emission Line of an Injection Laser", Soviet Journal of Quantum Electronics, Vol. 13(6), pp. 792-798, discloses various schemes of external optical resonator to provide feedback for the purpose of achieving the narrowing of the emission line or wavelength emission spectra of an injection or diode laser. One such scheme is shown in FIG. 1(d) of this reference employing an external resonator for an injection laser using a spherical mirror and reflective coating on regions of the laser facet adjacent to the point of beam emission from the laser. In seeking out the purpose of narrowing the laser wavelength emission spectra, E. M. Belenov et al employ an external cavity which provides a small amount of feedback. In this case, the length, L, between the laser facet and the spherical mirror is required not to be equal to the the radius of curvature of the spherical mirror, i.e., R.noteq.L. This scheme established an external cavity in which the Gaussian beam waist, W.sub.0, at the facet is much greater than the extent of the emitting active region. E. M. Belenov et al state that it should be comparable to or substantially equal to the size of the facet mirror. Also, the reimaged spot at the laser emission point aperture overlaps the latter to a great extent so that only a small amount of the feedback radiation is fed into the smaller laser emission aperture. This follows from the further fact that the Belenov et al article reports that R, the radius of curvature of the spherical mirror, should be greater than L, the distance between the laser facet and the plane of the spherical mirror, so that the beam waist at the facet, W.sub.0, is large. Also, the parameter a.sup.2, the feedback efficiency, for this particular embodiment is preferred to be a.sup.2 .ltoreq.10.sup.-2, which is a=0.1, i.e., a very low feedback efficiency (1% or less). As a result, there will be no focus, let alone high efficient, reimaging of the reflected beam back into the laser emission aperture since a majority portion of the reimaged beam will be out of the field or spot size of laser emission aperture. As a result, any significant sensitivity to misalignment relative to spatial relationship of the laser facet to the spherical mirror in a direction perpendicular to the axis, L, can only be realized by actually providing a unfocused beam as optical feedback in order to achieve a small amount of feedback for the purpose of changing the wavelength emission spectra with an additional advantage of lower sensitivity to misalignment within the limits of the larger feedback beam spot size on a much smaller emission aperture size.
The object of this invention is the provision of external optical feedback means designed for high efficient feedback of 90% or greater of the emitted radiation from the laser facet back into its aperture while being insensitive to limited misalignment of the laser facet relative to the spherical mirror to obtain high levels of optical feedback into the laser emission aperture without the need of fine tuned alignment.
This general object and other more specific objects of this invention will become apparent from the following description of this invention.